The need to take a class or two might be obvious, but seriously, where do you go for writing classes? Do you need an MFA in Creative Writing to write a good novel?
Yes and no. Yes, every writer needs to be invested in learning their craft. For some that means reading books on craft, for others it means taking seminars and going to writing conferences, and for still others it means butt-in-uncomfortable-plastic-chair, sit-at-table-with-strangers, and listen-to-lecturer for a few months a year.
No, you don't need to sign up for uncomfortable plastic chairs and student loans.
Yes, you need to learn your craft, and taking classes is a great way to do it.
Those professional writing organizations I mentioned earlier? They all have classes on craft. You don't have to spend a lot of money. For ease of use, I'll repeat some here:
~SCBWI (if you're writing children's and YA books): They have regional conferences that give valuable tools and information on the craft and business of writing for children.
~RWA (if you're writing romance): They have paid and free online classes every month, and local chapters have inexpensive walk-in seminars on craft and business of writing all fiction, not just romance. They are the most accessible, friendly professional writing organization I know of.
~SFFWA (Scifi & fantasy)
~HWA (horror)
~MWA & Sisters in Crime (mystery & crime writing)
~NINC (Novelists Inc)
Other sources for inexpensive, quality writing classes:
~Writer's Village University
~Savvy Authors
(I'm sure there are more, if you have a good resource feel free to add in comments.)